A large constructing dump truck as one form of a tipper has steerable fronts wheel and driving rear wheels mounted on a vehicle body, and also has a tiltable rear body. A typical known traveling system for such a dump truck mechanically drives the driving rear wheels by transmitting an output of an internal combustion engine, such as a diesel engine (hereinafter simple referred to as "engine") via a power transmission.
As a traveling system of a normal on-the-road vehicle, Japanese Unexamined Patent Publication (Kokai) NO. Showa 63-258223, for example, discloses a mechanical and hydraulic type four-wheel drive traveling system, in which an output torque of the engine is mechanically transmitted to the rear wheels via the power transmission, a variable hydraulic pump is driven by the output torque of the engine, and the front wheels are driven by a variable hydraulic motor driven by the discharged pressurized fluid of the hydraulic pump. Similarly, Japanese Unexamined Patent Publication No. Showa 61-191429 also proposes a four-wheel drive system for a vehicle, in which one of the front and rear wheels are driven by a hydraulic drive system.
In the traveling system having a drive system exclusively employing the mechanical drive system, a traction force can be lowered when slippage is caused on the driving rear wheels. Therefore, such drive system has low performance in traveling ability in a muddy area or in hill-climbing.
Conversely, with the four-wheel drive traveling system, in which the mechanical drive system and a hydraulic drive system are combined, traveling ability in muddy areas and hill-climbing performance can be improved. However, in such system, it becomes necessary to synchronize the rear wheels and front wheels. In the prior art, in detecting wheel slippage at the rear wheels, the displacement of the variable hydraulic pump and the capacity of the variable hydraulic motor are set at zero so as not to drive the front wheel, while no slippage is caused on the rear wheels, and the displacement of the variable hydraulic pump and the capacity of the variable hydraulic motor are varied to adjust the hydraulic force to a reference value to transmit the driving force to the front wheel.
Therefore, the displacement of the hydraulic pump and the capacity of the hydraulic motors are increased from zero after occurrence of wheel slippage on the rear wheels and varied against the certain hydraulic force as a reference value, to drive the front wheels. This causes time lag from occurrence of wheel slippage on the rear wheels to driving of the front wheels to make smooth traveling impossible.
This difficulty in providing smooth traveling is particularly significant in the case of a tipper having several of tens tons of movable load.
On the other hand, as set forth above, in the four-wheel traveling system, the wheel rotation speed on the front wheels is determined by the discharge amount of the variable hydraulic pump and the capacity of the variable hydraulic motor. Here, provided that the pump displacement is constant, the discharge amount of the hydraulic pump is determined by the engine revolution speed. Accordingly, the wheel rotation speed of the front wheels is variable depending upon the engine revolution speed. On the other hand, the wheel rotation speed of the rear wheels is determined by the engine revolution speed and a speed ratio of the power transmission. Therefore, provided that the engine revolution speed is constant, the wheel rotation speeds of the front wheels and the rear wheels may be differentiated at different speed ratios of the power transmission.
In order to solve the above-mentioned problem, the system disclosed in the above-identified Japanese Unexamined Patent Publication No. Showa 61-191429 is provided with an auxiliary hydraulic pump at the output side of the power transmission to supply the discharged pressurized fluid of the auxiliary pump to the variable hydraulic motor for varying the fluid amount to be supplied to the variable hydraulic motor depending upon the speed ratio of the power transmission so that the rotation speeds of the front and rear wheels are matched even at different speed ratios.
However, in such construction, the cost becomes high for installation of the expensive auxiliary pump, and piping becomes complicated.
Also, for four wheel driving travel in which rotation speed of the front and rear wheels is synchronized by controlling displacement of the variable hydraulic pump and the capacity of the variable hydraulic motor, the displacement of the variable hydraulic pump and the capacity of the variable hydraulic motor in the four-wheel drive traveling system are controlled for establishing synchronization with the wheel rotation speed on the rear wheels. In the above-mentioned four wheel drive type traveling system, when an abnormality or failure is caused in a mechanical transmission system mechanically connecting the engine and the rear wheels, such as failure of a torque converter or the power transmission, drop out or damaging of a propeller shaft, failure in a differential gear box and so forth, the rear wheel cannot be driven by the engine.
In this case, since the displacement of the variable hydraulic pump and the capacity of the variable hydraulic motor cannot be controlled, the front wheels cannot be driven hydraulically. Therefore, the vehicle cannot travel.
On the other hand, considering the shifting operation of the power transmission provided in the mechanical drive system, upon shifting, the engine revolution speed is abruptly lowered by releasing of the accelerator at initiation of shifting and the engine revolution speed is abruptly increased by depression of the accelerator after completion of shifting. Therefore, in shifting the speed ratio, the engine revolution speed is varied abruptly. Therefore, upon shifting, the discharge amount of the variable hydraulic pump is abruptly varied to cause abrupt variation of the supply flow rate for the variable hydraulic motor to cause abrupt variation of torque of the variable hydraulic motor to prevent optimization of the front wheel driving force to cause difficulty in smooth traveling.
For instance, assuming that the discharge amount of the variable hydraulic pump is set at a value capable of maintaining the torque of the variable hydraulic motor at a certain value during travel before shifting, the discharge amount of the variable hydraulic pump is lowered by the abrupt drop of the engine revolution speed upon shifting to make it impossible to maintain the torque of the variable hydraulic motor at the certain value to cause lowering of the driving force on the front wheels. When the decrease of the discharge amount of the variable hydraulic pump is significant, the hydraulic motor is driven in reverse by the inertia on the traveling vehicle through the front wheels to cause a pumping effect to increase resistance for revolution of the hydraulic pump to further lower the engine revolution speed.
On the other hand, in this kind of four-wheel drive type traveling system, it is possible to cause failure or fluid leakage in the variable hydraulic pump, the variable hydraulic motor and the closed circuit connecting the variable hydraulic pump and the variable hydraulic motor. When four wheel drive traveling is performed without knowing of a failure or fluid leakage, damaging of various portions and increasing of fluid leakage will likely be caused.